氣候變化 呈"鐘形曲線"變化的天氣

ARE heatwaves More common than they used to be? That is the question addressed by James Hansen and his colleagues in a paper just published in the Proceedings of the National Academy of Sciences. Their conclusion is that they are-and the data they draw on do not even include the current scorcher that is drying up much of North America and threatening its harvest. The team's method of presentation, however, has caused a stir among those who feel that scientific papers should be dispassionate in their delivery of the evidence. For the paper, interesting though the evidence it delivers is, is far from dispassionate.

熱浪來襲真的比以前更加頻繁了嗎？詹姆斯•漢森與他的同事試圖在《美國國家科學院院報》上發表的一篇新文章中回答這一問題。他們得出的結論是，情況的確如此，而且得出這一結論的依據這還不包括最近北美的酷熱天氣。現在那裏的大部分地區已經乾旱無比，當地農作物的收成也因此受到了威脅。然而，詹姆斯•漢森與他的同事所採用的展示方案卻在一羣人中引起了騷動。這些人認爲科技論文在表述證據時應客觀公正，但有意思的是，該文章在表述證據時卻遠未做到不偏不倚。

Dr Hansen, who is head of the Goddard Institute for Space Studies, a branch of NASA that is based in New York, is a polemicist of the risks of man-made global warming. Despite his job running a government laboratory, he has managed to get himself arrested on three occasions for protesting against those he thinks are causing such climate change. He clearly states in the paper's introduction that he was looking for a way of conveying his fears to a sceptical public.

總部在紐約的戈達德太空研究所(GISS)是美國國家航空航天局的一個分支部門，該研究所的負責人漢森博士能言善辯，他聲稱人爲因素導致的全球變暖將給人類帶來危害。儘管漢森博士主管政府實驗室，他卻參加過抗議活動，以此來反對那些他認爲將造成此類氣候變化的行爲，而他也因此"如願"被警察逮捕過三次。漢森博士在他的文章序言中明確指出，他正在尋找一種方法，以此讓那些對他的觀點持懷疑態度的公衆體會到他的焦慮。

Some of that scepticism is connected with the fact that although changes in the climate will inevitably result in changes in the weather, ascribing any given event-such as a local heatwave-to climate change is impossible. Dr Hansen has therefore tried to go beyond the study of individual causes by demonstrating that what was once unusual is now common.

儘管氣候變化將不可避免地導致天氣變化，但不能將所有特定事件（如地方性的熱浪）都歸因於氣候變化，而公衆的懷疑態度中有一部分就與這一事實有關。因此，漢森博士通過論證過去人們眼中的異常天氣如今已是十分常見，試圖使他的研究不僅僅侷限於個別原因。

Longer, hotter summers

夏天愈加漫長炎熱了

To do so, he and his colleagues took 60 years' worth of data (the period from 1951 to 2011) from the Goddard Institute's surface air-temperature analysis. This analysis divides the planet's surface into cells 250km (about 150 miles) across and records the average temperature in each cell. The researchers broke their data into six decade-long blocks and compared those blocks' statistical properties.

爲此，漢森博士與其同事從戈達德研究所收集了60年（從1951年至2011年）地表氣溫分析數據。這些分析材料將地球表面分成了一個個寬250公里（約150英里）的區塊，並記錄了每個區域內的平均溫度。研究者將這些數據按十年一組分成了六個時間段，並對這些時間段內的統計特性進行了比較。

They looked in particular at the three months which constitute summer in the northern hemisphere (June, July and August). First, they created a reference value for each cell. This was its average temperature over those three months from 1951-80. Then they calculated how much the temperature in each cell deviated from the cell's reference value in any given summer. That done, they plotted a series of curves, one for each decade, that showed how frequently each deviant value occurred.

研究者特別關注了北半球夏季三個月（六月、七月、八月）的溫度。首先，他們計算出了1951年至1980年每一個區塊夏季三個月的平均溫度，並將這些數據作爲每個區塊的參考值。接着，研究者又計算在任意指定的夏季內每個區塊內的溫度分別與各自的參考值存在多少偏差。這一步完成後，他們以十年爲單位繪製了六條曲線，這些曲線顯示出了每一個偏差值發生的頻率。

Since small deviations are common and large ones are rare, the result of plotting data in this way is a curve shaped somewhat like the cross-section of a bell. Such distributions can be modelled by a mathematical function known as the normal distribution-or "bell curve".

由於小的偏差時有發生，而大的偏差則較爲少見，因此用這一方法對數據進行描繪，得出的曲線形狀就像一個鐘的橫截面。這樣的分佈狀態可以用數學上的一個函數來表示，即正態分佈，又稱"鐘形曲線"。

Whether based on data or a mathematical ideal, such a curve always has two parameters. These are its mean (the value of all of the data points added together and divided by the number of points; this is also the peak of an ideal curve) and its standard deviation, which measures how wide the bell is. The standard deviation is calculated from all of the individual deviations of the data points.

無論是基於數據還是理想數學模型，鐘形曲線都有兩個參數。一個是平均值（將所有數據的值相加，除以總數所得；同時也是這條理想曲線峯值的橫座標值），另一個是它的標準差（代表鐘形曲線的"胖瘦"）。標準差可以通過計算所有數值的方差（再開方）得到。

To see what was going on, Dr Hansen superimposed the actual curves for each decade from the fifties to the noughties on a normal distribution, which acted as a reference curve. To make all the curves comparable, he expressed the values of the actual deviations as fractions of a standard deviation, and their frequencies as proportions of their total number.

爲了弄清氣候變化，漢森博士以正態分佈曲線爲參照曲線，將其與六十年中每十年一條的實際曲線相疊加。爲了使所有這些曲線具有可比性，漢森博士將實際偏差值用其與所屬樣本標準差的比例來衡量，用數據發生偏差數與數據總數的比例表述其頻率。

As the chart (right) shows, there are two trends. First, the peaks of the data-based curves move right, over time, with respect to the reference curve. In other words, the average temperature is rising. Second, more recent curves are flatter. A flatter curve means a bigger standard deviation and a wider spread of results.

如（右）圖所示，鐘形曲線表現出了兩種趨勢。第一，對照參考曲線，隨着時間的推移，這種數據型曲線的峯值會向右移，也就是說地球上的平均溫度在上升。第二，時間距離現在越近，曲線越"矮胖"，這也就意味着標準差越大，溫度數據分佈越廣。

If the mean of each curve were the same, such flattening would imply both more cold periods and more hot ones. But because the mean is rising, the effect at the cold end of the curves is diminished, while that at the hot end is enhanced. The upshot is more hot periods of local weather.

如果每一條曲線的平均值相同，那麼這種平滑過程也就意味着地球上的寒冷期和炎熱期的出現次數都會增加。但由於這些曲線的平均值在增加，因此地球上寒冷期的出現次數會減少，而炎熱期的出現次數則會增加。最終導致的結果就是各地出現炎熱天氣的次數越來越多。

Moreover, the bell-curve method makes it possible to say just how much more hot weather there is. Dr Hansen defined extreme conditions as those occurring more than three standard deviations from the mean of his reference curve. In that curve, this would be an eighth of a percent at each end, which is more or less the value in the curve for 1951-61. Nowadays, though, extreme conditions (or, at least, those that would have been considered extreme half a century ago) can be found at any given time in about 8% of the world.

此外，有了鐘形曲線，人們還能判斷炎熱天氣的出現頻率比過去增加了多少。漢森博士下了定義：如果在他的參考曲線中某一數值偏離其平均值達到或超過三個標準差，那麼這就是極端天氣。在參考曲線中（標準的正態分佈曲線），左右兩端數據中偏離均值超過三個標準差的極端數據應該恰好分別爲0.125%(百分之一的八分之一)，而1951至1961年這一時間段的極端數據分佈情況大致上也符合這一特徵。但現在世界上約有8%的地區在特定時間內都有極端天氣（或者，至少在半個世紀前會被認爲是極端天氣）的出現。

Local weather patterns do, of course, have local causes. To that extent, they are accidental. But Dr Hansen's analysis suggests that claims there is more hot weather around than there used to be have substance, too.

當然，各地天氣模式的出現的確包含着地方性的原因。在這種程度上說，它們的發生是具有偶然性的。但漢森博士的分析表明，各地的炎熱天氣的確比以往多這種說法也是有依據的。

Nothing in his analysis speaks of the cause of that substance. That is deliberate. As he says in the paper, he wants the data to speak for themselves-though he is personally convinced that the cause is human-generated emissions of greenhouse gases such as carbon dioxide. But as the United States bakes in what may turn out to be a record heatwave, he hopes he might now persuade those for whom global warming is, as it were, on the back burner, to agree that it is real, and to think about the consequences.

但他的分析並沒有涉及到這種依據的來源，而這也是漢森博士有意爲之。正如他在文章中所說，他希望數據能說明問題——儘管他本人相信氣候變暖是因爲人類排放二氧化碳等溫室氣體造成的。但現在，一股或許是有史以來最猛烈的熱浪正在炙烤着美國。他希望他現在能夠說服那些跟過去一樣擱置考慮全球變暖問題的人，使他們認同全球變暖現象的確存在並考慮其後果。